The present invention relates to electrodes for aluminum resistance spot welding, more particularly to improved electrodes for reducing the overall energy required to resistance spot weld aluminum.
Resistance spot welding is widely used for spot welding of steel and other metals, particularly in the assembly of automobile bodies and truck bodies. An apparatus for resistance spot welding includes a pair of resistance welding electrodes. Typically, a robot weld gun fitted with a pair of electrodes is moved in stages along a continuous weld path. At each stage, the electrodes are pressed against opposing sides of the work pieces to be welded, and an electric current is passed through the electrodes in the work pieces. The electrical resistance of the metal work pieces produces localized heating which causes the work pieces to fuse at a weld site. The electrical heating at the point of pressure between the electrodes forms a molten nugget at the interface between the work pieces. Typically, welds are oval in shape. The ovality of a weld nugget is dependent on the rigidity of the welding equipment and the condition of the electrodes. The diameter of a weld is considered to be an average value for the weld and is conventionally determined to be the average of the maximum diameter and the minimum diameter of the weld nugget.
Electrodes are typically made from copper or copper alloys to provide low electrical resistivity and allow high current flow therethrough. The tips of the electrode closest to the work pieces have geometrical configurations which are determined by the particular needs of the welding process. Typical geometrical configurations of an electrode tip include pointed, dome, flat, offset, truncated and radius. Electrodes with radius tips are substantially cylindrical with a generally convex face for contact with the work piece. Electrodes with truncated tips have a frustoconical shape with a generally flat face for contacting the work piece. As shown in FIG. 1, conventional electrode tips E are placed on opposing sides of a pair of metal work pieces M. The faces F of the electrode E contact opposing sides of the metal work pieces M. Upon application of electrical heat and pressure, a weld nugget W forms between the metal work pieces M.
The industry recommended specifications for equipment and process parameters for resistance spot welding aluminum is set forth in a publication by the Aluminum Association entitled xe2x80x9cT10 Guidelines to Resistance Spot Welding Automotive Sheetxe2x80x9d, hereinafter the xe2x80x9cAAT10 Guidelinesxe2x80x9d. The AAT10 Guidelines specify the minimum weld nugget diameter (DW) and electrode face diameter (DF) for a range of thickness (gauge) of aluminum sheet to be welded. Based on these specifications, the recommended ratio of the surface area of the weld (AW) to the surface area of the electrode face (AF) is about 0.2 to about 0.4 for radius electrode tips and over about 0.4 to about 0.7 for truncated electrode tips. This relationship is shown schematically in FIG. 1 where it can be seen that DF is substantially larger than DW.
Much of the resistance spot welding equipment in place today is designed for use in welding steel. It is desirable to weld other metals using this readily available equipment. For example, in the automotive industry, an increasing number of steel components are being replaced by aluminum. These aluminum components are preferably assembled using the welding equipment of their steel counterparts. However, aluminum has higher electrical conductivity and higher thermal conductivity than steel. Much of the heat generated in aluminum work pieces dissipates through the work pieces. As a result, resistance spot welding of aluminum using steel resistance spot welding parameters typically results in insufficient weld strength and inferior weld performance.
To overcome the heat losses due to the high conductivity of aluminum when using the electrodes of steel welding equipment, the amount of current passed between the electrodes typically is about three times the amount of current required to weld a similar gauge of steel. For example, resistance spot welding of steel typically requires about 10,000 to 15,000 amps, whereas resistance spot welding of aluminum conventionally requires about 20,000 to 50,000 amps. Hence, resistance spot welding of aluminum requires significantly higher current to achieve sufficient heat at the weld point between the electrodes than is required for welding steel. Production of these higher current levels requires heavier duty equipment which is more difficult to manipulate and operate using conventional robotics. Higher welding currents are detrimental the electrodes and decrease electrode life. Aluminum resistance spot welding is at a significant cost disadvantage as compared to welding of steel in terms of the expenses associated with higher current and shorter electrode life.
In addition, certain aluminum alloys such as those of the Aluminum Association alloy series 5xxx and 6xxx have heretofore been difficult to resistance spot weld. For example, resistance spot welding of 5xxx series alloys (such as alloys 5182-O and 5754-O) may exhibit expulsion and whiskering of the weld nugget. Resistance spot welding of 6xxx series alloys (such as alloys 6022-T4 and 6111-T4) is problematic due to the high thermal conductivity and electrical conductivity of 6xxx series alloys which renders them difficult to sufficiently heat to form an acceptable weld nugget at acceptably low currents.
Accordingly, a need remains for a method of resistance spot welding aluminum using conventional steel welding apparatuses at commercially acceptable power levels and resulting in acceptable electrode life.
This need is met by the apparatus and method of the present invention for resistance spot welding aluminum work pieces. The apparatus includes a pair of electrodes for positioning on opposing sides of a pair of aluminum work pieces to form a weld nugget between the aluminum work pieces with each electrode having a tip and a face. The improvement of the invention resides in the use of at least one electrode face which is sized so that ratio of the surface area of the weld nugget to the surface area of the electrode face is about 0.75 to about 1. Preferably, both electrode faces are sized so that the ratio of the surface area of the weld nugget to the surface area of each electrode face is about 0.75 to about 1. The diameter of the weld nugget (DW) is preferably about equal to the formula 5xc2x7t (five times the square root of t) where t is the gauge of the thinnest work piece being welded. The electrode tip is preferably truncated in shape.
The present invention also includes a method of resistance spot welding aluminum work pieces including steps of (1) contacting the opposing sides of a pair of aluminum pieces with a pair of opposing electrodes, each electrode having a tip with a face and (2) passing current from one electrode to the other electrode such that a weld nugget forms between the work pieces, wherein a ratio of the surface area of the weld nugget to the surface area of the electrode face is about 0.75 to about 1. The aluminum work pieces may be formed from alloys including but not limited to Aluminum Association 2xxx, 3xxx, 5xxx or 6xxx series alloys.